A heat-sensitive recording material utilizing a coloring reaction between a colorless or pale-colored basic dye and an organic or inorganic color developer by contacting the dye and the color developer through application of heat is well known. A heat-sensitive recording material of this type is widely used, for example, in a facsimile, a printer and as a recording medium for use in various calculators, because it is relatively inexpensive, a recording equipment containing it is compact and further is relatively easy to maintain.
Recently, either for the adaptability to multicolorization or for the use in an overhead projector (OHP), a transparent heat-sensitive recording material (whose support is a film) and the like with which recording can be performed directly by a thermal-head have been developed.
With extention of the application form, a heat-sensitive recording material is used under various conditions that the external circumstances are varied. Thus it is required for the heat-sensitive recording material to have excellent recording suitability so that a recording image can be obtained always stably. For this reason, various improvements have been proposed.
For example, when recording is performed under low humidity conditions, the friction between a recording equipment and a recording paper produces frictional charge, thereby decreasing the suitability for passing the recording paper and causing troubles such as paper plugging (jamming), sticking of the recording paper to the recording equipment, and break-down and abnormal or wrong operation of a thermal-head and other circuits. Therefore, a method of treating the recording material with various electroconductive substances such as metal oxides, metal halides, polymeric electrolytes, surfactants and hygroscopic substances is disclosed in, for example, JP-A-57-148687, JP-A-57-156292, JP-A-57-170794 and JP-A-57-199687. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In accordance with this method, a considerably high improvement can be obtained.
In recent years, in order to obtain a heat-sensitive recording material excellent in a degree of resolution, a film or synthetic paper has been increasingly used as a support. When such a support is used, even if various electroconductive substances as described above are used, satisfactory results cannot be always obtained. Thus still more improvements have been desired.
That is, when a film or synthetic paper is used as a support, in particular, frictional charging under low humidity conditions is marked as compared with the case that the ordinary paper is used as a support. Thus even if electroconductive substances as described above are used, no satisfactory charge preventing effect can be obtained.
It may be considered to use a metal-based electroconductive agent, but such a use is expensive and therefore not desirable because of economical reason. Besides, the metal-based electroconductive agent tends to be readily colored and, as a result, it decreases the commercial value of the obtained heat-sensitive recording material. If a large amount of such an electroconductive substance is used in order to increase the electroconductivity, problems are produced such that undesirable fogging is produced in the recording material and that coagulation likely occurs where the electroconductive substance is a polymeric electrolyte. Also, use of a large amount of a hygroscopic substance is attended by such problems that the recording material becomes tacky under high humidity conditions and that blocking between recording materials occurs.
On the other hand, from the standpoints of the use as a transparent heat-sensitive recording material (e.g., one for OHP) or the adaptability to multicolorization, it is a pressing need to develop a heat-sensitive recording material which has excellent color reproductivity and high degree of transparency.